An offshore production riser system usually includes multiple conduits through which various produced fluids (e.g. oil, gas, water, etc.) are transported between the seabed and the surface of the water body. Conduits also may be used for off-loading lines, fluid injection lines and service, electrical and hydraulic control lines.
It is known to lay a rigid or steel pipeline so it extends from a sub sea well or other fluid source on the sea floor using conventional submerged pipe laying techniques and curve one end of the pipeline upward in a gentle catenary path, or J-curve, through the turbulence zone and connect it directly to a submerged buoy or to a vessel floating on the surface. These rigid risers are commonly referred to as Steel Catenary Risers or “SCR's”.
A large field scenario may take years to fully develop, during which additional risers are occasionally laid to connect to wells. It is advantageous to minimize or eliminate the need for high cost special pipe lay vessels by self-installing the pipelines and risers from the Floating Production Unit (FPU). In this concept, a J-lay or workover rig is mounted on the deck of an FPU to deploy the pipeline at controlled, near-vertical angles. As the pipe string is deployed, it is pulled out by a direct pull (DP) tug. Instead of moving the rig and vessel relative to the pipeline, the FPU remains stationary as the pipeline is dragged outward toward a subsea well or tie-in location.
Alternatively a steel riser pipe can be assembled at a drilling rig, dragged away from it by a tug boat and be connected to a moored FPU or a sub sea well. These pipelines are assembled at the vessel and can for example be used as a seabed supported pipeline, a vertical riser, a Steel Catenary Riser (SCR) or as a steel midwater pipe arrangement which is supported by two floaters.
One pipe lay method is the J-lay, used especially in water of at least 600 meters depth, where pipe sections lying in a near vertical position are connected and moved down into the sea. It has been common to weld together the adjacent ends of each pair of pipes. It can take over a half hour to make weld connections and inspect them, and this adds considerably to costs. Applicant makes pipe connections by turning a threaded part to make a threaded connection. It takes about 15 minutes to make a threaded connection and move down the pipe string to ready it for another connection.
There are two major types of suitable threaded pipe couplings, and each has pipe sections with threaded ends. One type is a higher fatigue resistance coupling with at least the internally threaded pipe end being of enlarged outside diameter. An example of this is a weld-on type where male and female threaded sleeves are machined from thick forgings and welded to the ends of two pipe sections. A second type is a lower fatigue resistance coupling with threaded pipe ends of no greater outside diameter than the rest of each pipe section. An example is a non weld-on type where male threads are machined on the ends of two pipe sections and an intermediate threaded heavy wall forging sleeve screws the pipe ends together. Applicant notes that in some threaded connections of the higher fatigue resistance type, an upset forging is used to create a heavy walled internal pipe end with a threaded connection machined directly onto the upset end of the pipe. The weld-on pipe couplings require custom forgings, welds, machining of welds, and inspection of welds as by x-ray and/or ultrasonic techniques. Applicant estimates the cost for a 10 inch pipe weld-on coupling set ready to be transported to the ship for threaded coupling thereat, to be about $3000 to $5000. The cost for a non weld-on threaded sleeve type coupling for a 10 inch pipe is about $800 per coupling set. The difference between the two types is about $3000 per coupling set.
Generally, weld-on pipe couplings are useful in J-lay pipe strings for their high fatigue resistance. The upper region of the catenary portion of the pipe string is subjected to repeated bending as a floating body experiences heave, roll, pitch, drift, etc. as a result of waves, winds and currents. The lower region of the catenary portion of the pipe string also is subjected to repeated bending as the pipe string is repeatedly pulled up and laid down on the sea floor due to vessel movements. Weld-on pipe couplings can better avoid fatigue failure at these regions. If the pipestring should be damaged, a large section of the submerged pipeline has to be replaced, which is expensive especially in deep waters where divers cannot be used. A pipe string that could reliably avoid damage but which greatly reduced cost, would be of value.